Tread belt drives useful for propelling large machinery and other heavy mobile equipment which are forerunners of the present invention are shown in U.S. Pat. Nos. 2,530,379; 2,727,794 and 3,680,928. In those patents a drive system is described in which the number of links which are drive by the tumbler teeth at any given moment of time is less than the number of links which are wrapped about the tumbler. This system has the advantage of providing for even wear of the links and is achieved by having the pitch angle between the tumbler teeth larger than the pitch angle of the links.
The pitch angle of the tumbler teeth is the angle at the center of the tumbler which is subtended by a line drawn from the midpoint of one tooth to the midpoint of an adjacent tooth. The pitch angle of the links is the angle at the center of the tumbler that is subtended by a line drawn from the center of the pivot pin connection of one link to the pivot pin connection of an adjacent link. A detailed discussion of pitch angles and their measurement appears in U.S. Pat. No. 2,727,794.
The described prior art type of tread belt drive system provides a practical and effective way of propelling large excavating equipment. However, the use of such tread belt drive systems is not without problems. One problem which can occur is that of breakage of the links or the teeth of the drive system. This can happen when a link being driven by a tumbler tooth is forced radially outward of the tumbler in response to the driving forces. When this happens there can be a loss of proper driving engagement between the tumbler tooth and the link and as the link falls back towards the tumbler the link may improperly strike a tumbler tooth and cause a fracture of either one or both of the members making repairs necessary.
Another problem which can occur is that due to the wear of the bearing surfaces of the tumbler and the links. Such wear can cause the pitch angle of the links to increase and the pitch angle of the tumbler to decrease. When the pitch angle of the links exceeds the tumbler pitch angle the necessary driving relationship of the tumbler to the links is lost and the links or tumbler must be rebuilt or replaced.
Still another problem which can occur is the jamming or breaking of the tread belt when stones or dirt become lodged between the driving surfaces of the tumbler teeth and the link.
Despite the above described problems and the fact that tread belt drive systems are generally noisy and rough operating, tread belt drive systems remain the most practical way of propelling large excavators and other heavy mobile machinery. Therefore, a need exists for an improved tread belt drive system which provides a smoother, quieter operation and eliminates the problems which accompany the use or prior drive systems.